The effect of different chemical treatments on the structure and stability of aqueous dispersion of iron- and iron oxide-filled multi-walled carbon nanotubes

Abstract

O efeito de cinco diferentes tratamentos químicos (HNO 3 , H 2 SO 4 , H 2 O 2 , HNO 3 + H 2 SO 4 e HNO 3 + HCl) na homogeneidade, superfície, estrutura e dispersabilidade em água de nanotubos de carbono do tipo multi-paredes preenchidos com ferro ou óxido de ferro foi estudado através de difratometria de raios X (XRD), espectroscopias Raman, UV-Visível e fotoeletrônica de raios X (XPS), análise termogravimétrica (TG) e microscopia eletrônica de varredura (MEV). Os resultados indicaram que os tratamentos são efetivos na remoção de espécies carbonáceas diferentes de nanotubos, presentes na amostra. Com exceção do tratamento com H 2 O 2 , uma boa remoção das espécies contendo metal também foi observada. Além de aumentar a homogeneidade das amostras, os tratamentos também criam grupos carboxílicos e hidroxílicos superficiais, que afetam diretamente a dispersabilidade destas espécies em água. Dispersões estáveis de 2,24 × 10 -1 g L -1 de nanotubos em água foram obtidas após o tratamento com mistura de ácido sulfúrico e ácido nítrico.The effect of five different chemical treatments (HNO 3 , H 2 SO 4 , H 2 O 2 , HNO 3 + H 2 SO 4 and HNO 3 + HCl) on the homogeneity, surface chemistry, structure and dispersibility in water of ironand iron oxide-filled multi-walled carbon nanotube samples was evaluated by X-ray diffractometry (XRD), Raman, UV-Visible and X-ray photoelectron (XPS) spectroscopies, thermogravimetric analysis (TG) and scanning electron microscopy (SEM). The results indicate that the chemical treatments are generally effective in removing non-nanotube carbonaceous species present in the sample. With the exception of the H 2 O 2 treatment, the chemical treatments also offer good removal of free iron-species. Besides the increase in the sample homogeneity, the chemical treatments promoted an increase in the carboxyl and hydroxyl groups at the carbon nanotube surface, what directly affects the dispersibility of these carbon nanotubes in water. Dispersions of 2.24 × 10 -1 g L -1were obtained for the treated samples with a mixture of nitric and sulfuric acid. Keywords: carbon nanotubes, chemical treatment, CVD, nanostructures, nanomaterials IntroductionCarbon nanotubes (CNTs) have received much attention in the last years due to their extraordinary chemical and physical properties, arising from the combination of their typical morphology, structure and size.1 Many different processes have been described for the synthesis of CNTs. Among them, one of the most versatile is the catalytic chemical vapor deposition (CVD), which is based on the decomposition of a carbon source (usually a hydrocarbon) over metal nanocatalysts.2 Usually, the resulting samples are a mixture of CNTs and some impurities, such as other carbonaceous materials (graphite, amorphous carbon, fullerenes, carbon nano-structures, etc.) and the catalyst metal particles.3 These impurities are undesirable for a variety of applications, and different physical and/or chemical treatments are usually performed to remove them. 4 However, it should be noted th...

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